add section on using non ssr-ready components

[antony]

Fixes #44

[mindrones]

I think it's not immediately clear how to use the component so I'd add something like:

async oncreate () {
  const MyComponent = await import("../path/to/MyComponent");
  this.set({MyComponent})
}

and mention that you use it as:

<svelte:component this={MyComponent}
  {prop1}
  prop2="foo"
/>